[Oz Cello]

You can just go the new library and import the pdf from the folder that MSP stores it in for the original library but that just gives you the pdf without any settings or parameters that you might've applied to it in the original library. When I want to keep those settings I export it as an msf file in the old library and then open the new library and import the msf file. That way all the MSP settings and options (annotations, collections/artist/composer/difficulty/etc, display settings etc) are retained. When I import the msf file MSP tells me that the file already exists and asks if I want to use the original file or make a new one. If you use the original file then it avoids doubling up but still allows you to have different parameters for the same pdf in each library.
Let me know if you find a better way though.
Guy